1. Field of the Invention
This invention relates to the reduction of phase noise in a multichannel radio receiver where the multiple channels share a common clock.
2. Description of Related Art
One example of a radio receiver with multiple channels which share a common clock is a receiver adapted to receive the signals of the NAVSTAR Global Positioning System (GPS) satellites. The GPS system consists of a number of satellites orbiting the earth. Each satellite contains a precise oscillator which provides the basic clocking signal for that satellite. Each satellite broadcasts one or more carrier signals each of which is modulated by a set of pseudo-random code sequences unique to that satellite. Each satellite broadcasts modulated carrier signals at the same set of frequencies, typically called L1 and L2, because the satellites broadcast in the L band of frequencies.
A typical GPS receiver consists of one or more separate channels, each channel being assigned to tracking one of the satellite signals available at the receivers position. Most modern GPS receivers have the facility to track the recovered carrier phase of the satellite signal by use of either a Phase Locked Loop (PLL) or Costas Phase detection loop (Costas Loop). The bandwidth of these tracking loops directly affect the ability of the receiver to acquire or track signals during periods of high receiver dynamics or in the presence of a jamming signal.
Oscillator phase noise in the receiver is a basic limitation on narrowing the carrier tracking loop bandwidth and, therefore, on the achievable carrier-track C/N.sub.0 for a GPS receiver tracking a single satellite signal, where C/N.sub.0 is the ratio of carrier signal to noise relative to a one Hertz bandwidth. However, when receiving several satellite signals, the receiver phase noise is common to all tracking loops and, in principle, can be removed by a common-mode rejection scheme. The phase noise contributed by the satellites is negligible in comparison with the phase noise contributed by the receiver's oscillator; hence, the common-mode rejection can be improved by tracking multiple satellite signals.
Traditionally the method for reducing the common phase noise in a GPS receiver is to use a higher quality and by extension higher cost oscillator. Use of the disclosed method allows a GPS receiver to achieve higher performance with an existing oscillator or to maintain a desired performance while allowing the use of lower cost crystal oscillators.